Alloy steel



April 30, 1940- K. T. BERGLUND 2,199,096

ALLOY STEEL Filed April 28, 1938 K ar! Tor/fe! B erg/u d Patented 'Api'.30, 1940 UNITED STATES ALLOY STEEL Karl Torkel Bergland,

Sandvken, Sweden, as-

\ I signor to Sandvikens Jernverks Aktiebolag,

Sandviken, lSweden, a. company of sp1-112s, 1938, serial No. 204,153Sweden April 30, 1937 y Application In Sweden 2 Claims. (Cl. 'l5-126)'Ihe present invention relates to an alloy steel which is capable ofbeing hardened to martensitic structure, and which is particularlyadapted to be used for the manufacture of corrosion resistant hardenedarticles, such as pointed tools and tools having a cutting edge. It isan objectof the invention to produce an alloy steel which has aconsiderably lower hardening temprature for attaining optimum hardnessthan chromium steels hitherto used for the purposes above mentioned.This object is attained, according to the invention, by incorporating inthe steel the alloy components nitrogen and molybdenum and/or tungsten.

'Ihe alloy steel according to the present invention is principallycharacterized by that it contains from 0.50 to 2.50% carbon, from 8 to22% chromium, from 60 to 90% iron, from 0.050 to 0.25% nitrogen and atleast 0.20% molybdenum and/or tungsten, and normal percentages ofmanganese, silicon, phosphorus and sulphur contained in commercial ironand steel.

Corrosion resistant chromium steels that are capable of being hardenedto martensitic structure are, generically speaking, built up on an alloybase the composition of which lies within the following approximate Percent Carbon 0.20- 2.50 Chromium 8 -22 Iron, maximum about 90 The carboncontent that is most suitable for different fields of use is determined,in the main, by the maximum hardness that it is sought to attain byhardening, and also by the desired quality and properties of the edge ofthe tool. If the edgeis required to possess a high degree of hardnessand a long life,..the carbon content shou1dbe at least 0.50%, andsuitamy not lower than 0.70%. To make it possible to work the steel in acold state by cold rolling or cold drawing, the carbon content shouldnot exceed 1.35% or 1.40%. The most suitable chromium content lies ingeneral between 13% and 18%.

The characteristic hardening properties of a steel of v this known typemay be exemplified by a steel of the following analysis:

Per cent Carbon 1.23

Manganese 0.43 Silicon 0.07

' Chromium 13.4

Nitrogen 0.021

Test plates made of this steel and measuring about 4 by 30 by 25 mm.were iirst heated during live minutes in a salt bath at differenttemperatures and. were afterwards hardened in oil. The hardness of thesesamples was determined in a Vickers hardness testing machine at a. loadof l 30 kg. The dependence of hardness upon the hardening temperature isillustratedl by the curve I' in Ithe diagram on the attached drawing inwhich the horizontal line represents temperatures and the vertical linerepresents hardness. 10

In this case optimum hardness is obtained when hardening at atemperature of 1050f C. The shape of the curve shows that optimum.hardness can only be attained within Aa very l limited temperaturerange. The maximum 16 hardness that can be attained with a chromiumsteel of this'type is considerably lower thanthe hardness that isobtained by hardening a nonalloy steel having the correspondingpercentage of carbon. Forl this reason it is as a rule very 20important, when hardening a chromium steel of this type, that themaximum hardness is really attained. The hardening temperature requiredfor this purpose, in this instance 1050 C., is in many casesinconveniently high. Most electric hardening furnaces used in theindustry are provided with resistance elements of chromiumnickel or asimilar alloy which, having regard to the required duration,particularly in the case of large furnaces, may hardly be used at highertemperatures than about 1000 C. In practice, therefore, it is notpossible in such a furnace to harden a simple chromium steel of thistype to maximum hardness.

Extensive experiments with vdifferent alloy components have proved thatthe hardening temperature required for attaining maximum hardness islowered by increasing the percentage of nitrogen to a vvalue whichconsiderably exceeds. the normal amount of the said substance whenpresent as an unintentional impurity, which a great number of analyseshave proved lies usually between 0.010 to 0.020% and on1y`exceptionallyrises to about 0.030%.

As an example of a steel having a nitrogen 1 content higher than normal,a s teel of the following analysis is given:

Per cent Carbon 0.96 Manganese 0.48 Silicon 4 0.16 Chromium 13.3Nitrogen` 0.145

The dependence of hardness upon the harden- 55

